When Mitt Romney speaks on Thursday night, the lights will go down, the delegates will hush and the atmosphere will build. It happened when his running-mate Paul Ryan came to the podium on Wednesday. But so far, the rest of the time – the long hours before television's prime time inside the hall and the longer daylight hours outside it – there has been something curiously lacking from this Republican convention, an element veterans of these quadrennial shindigs once took for granted: call it buzz, energy or atmosphere … whatever it is, Tampa doesn't have much of it.

"This is the most low-energy convention I've ever been at, by far," says one of America's most visible political journalists, present at every nomination gathering of the last 20 years (but keen to remain anonymous lest he compromise his public neutrality). Conversations with other seasoned observers run on similar lines. Even Michael Barone, the conservative-leaning editor of the Almanac of American Politics told the Guardian that this, his 21st convention, was among the most "subdued" he had attended.

The evidence is found in the lack of crackle in the hall, the constant murmur and hubbub that persists for all but the top-of-the-bill speeches, the way the usual trigger for tear-the-roof-off cheering – the merest mention or glimpse of the nominee – instead goes by unnoticed. The euphoria that coursed through Dallas in 1984, when Republicans nominated Ronald Reagan, or through Denver in 2008, when Democrats selected Barack Obama, is missing from Tampa.

Some in the hallways blame Isaac, the hurricane that blew away the first day of the convention and which had organisers fearing either total cancellation or, if not that, the pressure to tone things down lest Republicans be seen partying while the Gulf Coast battled the elements. And yet, Hurricane Gustav axed the first day of the Republican convention of 2008, too, but that gathering was electric – powered by the shock nomination of Sarah Palin as John McCain's running-mate.

Others say the problem is not the extreme weather, just the regular climate of Tampa in August: hot, humid and sweaty, in the high 80s even at midnight. There's no denying that when even a short walk feels like a "shower in molasses", as Twitter puts it, that can dampen the spirits. But the temperature in Dallas 1984 apparently reached 107F – yet delegates skipped into the hall to nominate Reagan.

Perhaps the problem is the security, so heavy that central Tampa has been dubbed the Green Zone. The military, police and secret service manning the endless miles of steel fencing are polite, but the consequence is that travelling around can feel like an ordeal.

"It's a pain in the butt," says Daniel R Centinello, a Republican delegate from New York state. Staying in a hotel several miles outside Tampa – because the city itself doesn't have enough hotel rooms – Centinello's journey to the convention has taken two and a half hours each way. That saps the enthusiasm somewhat. There was heavy security at the post 9/11 2004 Republican convention in New York City, too, but those who were there say getting around is harder this time.

Still, none of this gets to the heart of the matter. The truth is that unless the power source is a high-voltage bust-up – as when Democrats split at the height of the Vietnam war in Chicago 1968, or Republicans were riven by Pat Buchanan's call for a "culture war" at Houston 1992 –the energy level of a nominating convention is determined by the energy flowing from and towards the nominee. Both New York in 1992 and Chicago in 1996 rocked because of Democrats' enthusiasm for Bill Clinton, just as Philadelphia 2000 drew its strength from Republican zeal for George W Bush.

But Tampa's Republicans struggle to feel that fervor for Mitt Romney, even as increasing numbers of them believe he has a genuine shot at winning in November. When his name is mentioned or his face appears on a screen, there are no foot-stamping cheers. Indeed, mention of Ryan tends to produce the bigger response – his Wednesday speech giving a welcome shot of adrenalin to proceedings.

"Romney's a Massachusetts moderate and a Mormon," says that seasoned reporter. No wonder the Republican faithful don't throb with love for him.

"This is certainly a less royal convention," is how Barone puts it. While Reagan, Clinton and Obama enjoyed coronations, this is a more down-to-earth meeting, lacking that intense focus on a celebrated, even loved, individual. It is, perhaps, rather like the countless business conventions – of the insurance industry or accountancy sector – that take place in American cities every week of every year. Come to think of it, that atmosphere might suit Mitt Romney rather well.